Preheat your oven to 180'c. Add a tablespoon of oil into a large non stick frying pan. Fry your onions and frozen veg until onion is translucent. Let cool slightly.Beat the eggs in a bowl and add cooled veg, cheeses and tomatoes, season with pepper. Put your frying pan back onto the heat and pour in your mixture. Stir around gently with a spatula until your start to feel the eggs setting at the bottom of your pan. then turn off the heat. Place frittata into the oven and let cook for 5-8mins or until eggs have set and cheese melted. Slide onto a serving board and serve hot (or cold if you're like me)